Rayleigh fading is a phenomenon associated with radio communications where multiple reflections of a transmitted signal constructively and destructively superpose at points in space according to the relative phase and amplitude of all the reflected signals. This spatial interaction of multipath signals produces both strong peaks (local maximums) and deep nulls (local minimums) of relative signal strength with regard to varying position. Often the fading can result in signal "holes" where the signal strength will drop below a minimum threshold of reception of a radio receiver, e.g. a mobile radiotelephone. Depending on the relative phase of the interacting signals, the peaks and nulls of signal strength are generally separated by a fraction of the free space wavelength of the carrier frequency of the transmitted signal. These signal strength fluctuations present a significant and challenging problem particularly in the mobile radio communications environment.
One mechanism which addresses the problems of Rayleigh fading is a space diversity technique where two or more antennas are separated by a minimum of one half of a wavelength of carrier frequencies to be received. At UHF frequencies, for example, a deep signal fade can be overcome by moving the antenna only a few inches. By separating two antennas by the appropriate distance, each antenna receives a signal whose fading pattern is uncorrelated with the fading pattern of the signal received by the other antenna. Improved signal reception is then achieved simply by programming the receiver to select (using one or more conventional techniques) the antenna with the strongest signal. More sophisticated techniques combine the signals from both antennas.
Recently, space diversity antennas have been considered for use on portable radiotelephones. For a telephone installed in an automobile which moves rapidly in and out of signal fades, Rayleigh fading is usually a transient and therefore tolerable occurrence. However, handheld portable radiotelephones are often operated with relatively slow moving (even stationary) characteristics such that if a user is positioned at a "deep null". reception (if any) is poor. Consequently, Rayleigh fading is particularly troublesome for portable radiotelephones.
Unfortunately, the dimensions of a portable radiotelephone, being comparable to that of a fraction of the free space wavelength at UHF frequencies, make the addition of a plurality of antennas difficult. A great selling point of portable telephones is of course their compact size and sleek appearance. Adding two antennas to a compact and sleek structure without adversely impacting these characteristics is a difficult task. In fact, the resulting ungainly appearance and inconvenient deployment of two antennas are primary reasons that in spite of the statistical improvement to signal reception by reducing sensitivity to Rayleigh fading provided with spatial diversity, most consumers dislike the appearance of diversity antennas on a portable radiotelephone.
One approach to overcome this appearance problem is to install two antennas within the body of the radiotelephone. Although this approach suitably addresses the problem of consumer appeal and convenience, it requires the use of compact antennas which suffer from reduced gain, and therefore, poor signal reception. Further, antennas installed in the body of the portable radiotelephone are typically blocked by the user's hand or head while listening or talking.
It is therefore an object of the present invention to provide for an improved diversity antenna for use in a portable radiotelephone which offers both high gain, consumer convenience, and an appealing appearance. It is another object of the present invention to provide for an improved diversity antenna for use in a portable radiotelephone that is not blocked by the user's head or hand. It is yet a further object of the present invention to provide for a diversity antenna for use in a portable radiotelephone that can be easily deployed in a single motion.
An adjustable diversity antenna assembly for use with a portable radiotelephone includes a plurality of antennas connected at one end to the portable radiotelephone for receiving radio frequency signals. A non-conductive member connects and spaces the plurality of antennas in a deployed position to reduce Rayleigh fading effects. The antennas are spaced apart in their deployed position by the connecting member which has a length essentially the same as some fraction, e.g. a quarter, of the wavelength of the received radio frequency signals.
The connecting member is used to deploy and stow the antenna assembly on the portable radiotelephone from an extended deployed position to a compact stowed position. Moreover, the diversity antenna assembly may be stowed in a cavity formed in the body of the portable radiotelephone.
The member and the plurality of antennas may be formed as an integral body made of molded plastic which forms the member and encases the antennas. In one embodiment, the integral body is a rotatably connected cover for covering a front face of the radiotelephone when the antenna assembly is stowed. In another embodiment, each of the antennas is a telescoping antenna which is moved to the deployed position by pulling the member away from the radiotelephone and stowed by pushing the member toward the radiotelephone.
In another embodiment, the portable radiotelephone according to the present invention includes a transceiver and a plurality of antennas connected to the transceiver for receiving transmitted signals. An axially adjustable non-conductive member connects the plurality of antennas. Means are included for laterally extending at least one of the plurality of antennas .